1. Field of the Invention
The present invention relates to a solid electrolyte fuel cell and more particularly to a solid electrolyte fuel cell having a stack construction with improved thermal reliability.
2. Description of the Prior Art
Fuel cells using oxide solid electrolyte such as zirconia, which operate at high temperatures such as 800.degree. to 1,100.degree. C., not only exhibit high power generation efficiencies but also need no catalyst. Such fuel cells are easy to handle since the electrolytes used are solid. For this reason, it is expected that they will be employed as third generation fuel cells.
However, the conventional solid electrolyte fuel cells are difficult to realize since they are susceptible to thermal damage because of the use of ceramics as their main construction components and the lack of a suitable method for providing seals for gases. Fuel cells having special forms, e.g., a cylindrical form, have been designed to solve the above-described problems have been and run successfully. However, the cylindrical fuel cells gave low power generation densities per volume of unit cell, and there has been no indication that they can provide economically advantageous fuel cells.
FIG. 1 is an exploded perspective view showing a conventional planar or plate-like solid electrolyte fuel cell. As is well known, it is necessary to make fuel cells planar in order to increase power generation density. A fuel cell of this type includes a unit cell 17, and a separator plate 18. The unit cell 17 includes a solid electrolyte plate 17A which is of ceramics and electrodes 17B and 17C. Two separator plates 18,18 of ceramics and one unit cell 17 are alternately superposed one on another. In other words, the unit cell 17 is sandwiched by the two separator plates 18, 18. The separator plates are each formed with first grooves and second grooves on different surfaces, running at right angles to each other, through which grooves different reaction gases flow, respectively. The reaction gases are introduced respectively thorough gas manifolds (not shown) into the fuel cell.
For, example, in order to feed the reaction gases sufficiently and separately into the fuel cell, it becomes necessary to prevent leakage of gases between the unit cell 17 and the separator plate 18 around the periphery of the unit cell. To this end, it would seem to be a possible solution to render the unit cell 17 and the separator plate 18 integral by sintering them together. However, this method would be unsuccessful since the unit cell and the separator plate are-made of different materials and thermal stress could appear which would lead to cracks in the sintered body if there is even a slight difference in coefficient of thermal expansion or nonuniform temperature distribution between the materials. Accordingly, is has been contemplated to prevent leakage of gases using sealing materials.
Japanese Patent Application Laying-open No. 267869/1990 discloses a solid electrolyte fuel cell which comprises a unit cell having a solid electrolyte on whose main surface are arranged an anode and a cathode, and first and second substrates sandwiching the unit cell and feeding reaction gases thereto, the resulting structure being superposed via an interconnector, in which (1) the first and second substrates have guide vanes arranged thereon that guide the reaction gases from the central parts toward the peripheral parts of the main surfaces, (2) the fuel cell includes reaction gas inlet pipes penetrating a stack composed of the first and second substrates, the unit cell, and the interconnector, in the central part thereof in the direction of the stack, the pipes being formed with gas ports on side surfaces thereof and for diffusing the gases toward the guide vanes, and (3) a glass seal is provided in spaces between the reaction gas inlet pipes and the inner periphery of the unit cell.
Japanese Patent Application Laying-open No. 168568/1990 discloses a solid electrolyte fuel cell having a construction similar construction to that described in the above-cited publication in which a glass seal is provided in spaces between the first and second substrates and between the reaction gas inlet pipes as reaction gas feeding means. Optimally, a glass seal may be provided in spaces between the outer peripheral surface of the unit cell, and spaces between the outer peripheral surface and the reaction gas feeding means.
Japanese Patent Application Laying-open No. 75262/1992 discloses a solid electrolyte fuel cell which comprises a plurality of first ribbed porous substrates each having on one surface thereof a unit cell, a plurality of second ribbed porous substrates each having on one surface thereof an interconnector, the first and second ribbed substrates being superposed one on another alternately, and first and second manifolds for feeding fuel gas and oxidant gas, respectively. The fuel cell further comprises a gas impermeable layer and a gas seal part. The unit cell is composed of three layers, i.e., an anode, a solid electrolyte and a cathode. Each first ribbed porous substrate has ribs which guide the fuel gas from the first manifold toward the peripheral part thereof on a surface opposite to the surface on which the unit cell is superposed. On the other hand, the second ribbed porous substrate has ribs which guide the oxidant gas from the second manifold toward the peripheral part thereof on a surface opposite to the surface on which the interconnector is superposed. The gas seal part is composed of a silver O-ring which is provided around the oxidant gas feeding manifold in the first ribbed porous substrate, and around the fuel gas feeding manifold in the second ribbed porous substrate. The gas impermeable layer is an O-ring made of glass or ceramics and intervenes between the gas seal parts and the ribbed porous substrates. Gas seal parts may be provided around the oxidant gas feeding manifold part in the anode and around the fuel gas feeding manifold part in the cathode, on the outer peripheral part of the first ribbed porous substrate, etc., as necessary.
The use of the seal materials requires further improvement in the stability of the gas seal since both unit cells and separators are made of ceramics.